Here's example.
- Alice enters USA on 01/01/2020 on CR-1 visa after 1.5 years of living separately from spouse Bob, because he's in the US, she's back in France getting visa.
- Alice is a resident since 01/01/2020
- Alice's 3 year anniverary as LPR is on 01/01/2023
- Alice files based on 3 year rule, marriage to US citizen. She also uses 90 day early filing window, so N-400 is filed on 10/03/2022.
- N-400 is denied due to not meeting marital union requirement. Yes, she's married for over 3 years. But marital union counts as 3 years from 01/01/2020 and there is no early filing window for that. At time of filing you must live together, physically for 3+ years.
- If Alice filed on 01/01/2023 or later, she would have been approved